Age-related macular degeneration (AMD) is the leading cause of severe visual loss in the developed world (Taylor et al., 2001; VanNewkirk et al., 2000). In the early stages of the disease, before visual loss occurs from choroidal neovascularization, there is progressive accumulation of lipids in Bruch's membrane (Pauleikhoff et al., 1990; Holz et al., 1994; Sheraidah et al., 1993; Spaide et al., 1999). Bruch's membrane lies at the critical juncture between the outer retina and its blood supply, the choriocapillaris. Lipid deposition causes reduced hydraulic conductivity and macromolecular permeability in Bruch's membrane and is thought to impair retinal metabolism (Moore et al., 1995; Pauleikhoff et al., 1990; Starita et al., 1996). Retina and/or RPE may respond by elaboration of angiogenic factors (e.g. VEGF, vFGF) that promote growth of choroidal neovascularization.
Interestingly, lipid accumulation in Bruch's membrane similar to that in AMD has been observed in apolipoprotein E (apo E) null mice (Dithmar et al., 2000; Kliffen et al., 2000). Because of the additional association between apo E alleles and other age-related degenerations, Alzheimer's disease and atherosclerosis, there has been recent investigation into a potential role for apo E in AMD.
Several studies on apo E polymorphism in AMD have been conducted (Simonelli et al., 2001; Klaver et al., 1998; Souied et al., 1998). In contrast to Alzheimer's disease, the apo E-4 allele has been associated with reduced prevalence of AMD. Apo E-2 allele is slightly increased in patients with AMD. Further supporting a role in AMD pathogenesis, apo E has been detected in drusen, the Bruch's membrane deposits that are the hallmark of AMD (Klaver et al., 1998; Anderson et al., 2001). Immunohistochemistry on post-mortem eyes has demonstrated apo E in the basal aspect of the retinal pigment epithelium (RPE) (Anderson et al., 2001). Cultured RPE cells synthesize high levels of apo E mRNA, comparable to levels found in brain (Anderson et al., 2001).
While the role of apo E in AMD is not established, this apolipoprotein has several functions that may affect the course of this disease. Apo E has anti-angiogenic (Browning et al., 1994), anti-inflammatory (Michael et al., 1994), and anti-oxidative effects (Tangirala et al., 2001). These are all considered atheroprotective attributes of Apo E, but may also be important in protecting against progression of AMD. While atheroprotective effects of apo E were initially thought to stem from effects on plasma lipid levels, local effects on vascular macrophages are probably equally important. Thus, selective enhanced expression of macrophage apo E in the arterial wall reduces atherosclerosis in spite of hyperlipidemia (Shimano et al., 1995; Bellosta et al., 1995; Hasty et al., 1999). Conversely, reconstitution of apo E null macrophages in C57BL/6 wild type mice induces atherosclerosis (Fazio et al., 1994). Atheroprotective effects of arterial apo E expression are thought to derive in part from facilitation of reverse cholesterol transport (Mazzone et al., 1992; Lin et al., 1999). The mechanisms by which apo E facilitates reverse cholesterol transport are incompletely understood. Apo E expression increases cholesterol efflux to HDL3 in J774 macrophages (Mazzone and Reardon, 1994) and lipid free apolipoprotein A1 (Langer et al., 2000). Cell surface apo E is also hypothesized to induce efflux from the plasma membrane (Lin et al., 1999).
Reverse cholesterol transport may be important in the pathogenesis of AMD because of lipid efflux from RPE into Bruch's membrane. Very much like intimal macrophages, RPE cells progressively accumulate lipid deposits throughout life; however, unlike vessel wall macrophages, the source of RPE lipid is thought to be retinal photoreceptor outer segments (POS) (Kennedy et al., 1995). Every day, each RPE cell phagocytoses and degrades more than one thousand POS via lyzosmal enzymes. These POS are enriched in phospholipid and contain the photoreactive pigment, rhodopsin. Incompletely digested POS accumulate as lipofuscin in RPE. By age 80, approximately 20% of RPE cell volume is occupied by lipofuscin (Feeney-Burns et al., 1984).
Analysis of Bruch's membrane lipid reveals an age-related accumulation of phospholipid, triglyceride, cholesterol, and cholesterol ester (Holz et al., 1994; Curcio et al., 2001). The origin of these lipids also is thought to derive principally from POS rather than from the circulation (Holz et al., 1994; Spaide et al., 1999). POS lipids are hypothesized to efflux from the RPE into Bruch's membrane. Although cholesterol ester deposition in Bruch's suggests contribution from plasma lipids, biochemical analysis of these esters suggests esterification of intracellular cholesterol by RPE cell derived ACAT (Curcio et al., 2002). While trafficking of lipids from the retina to RPE cells has been studied extensively, mechanisms of lipid efflux from RPE to Bruch's membrane are not well understood. Furthermore, from a pathogenic standpoint, regulation of lipid efflux into Bruch's membrane may be important in determining the rate of lipid-induced thickening that occurs in aging.
In AS, similar to AMD, lipids accumulate in the extracellular matrix and within phagocytic cells, primarily macrophages. Mechanisms of lipid metabolism in AS have been investigated in detail. Similar investigations into lipid processing by RPE and subsequent lipid efflux into BM and the circulation have not been conducted with the same depth as those for AS. As a consequence, potential therapeutic approaches to dry AMD are wonting.
Navab et al. (2003) describe ApoA-I mimetic peptides comprising D-amino acids for oral delivery for the treatment of atherosclerosis.
U.S. Patent Application Publication US 2002/0142953 relates to human compositions encoding apolipoproteins that are related to lipid metabolism and cardiovascular disease.
Thus, the present invention provides a novel approach to reduce lipid content of ocular tissue, such as Bruch's membrane and further provides methods and compositions for the treatment of macular degeneration, such as AMD.